Medical X-ray transparencies usually are examined by placing them over the faceplate area of a device commonly referred to as an illuminator (or viewbox). Conventional illuminators normally comprise a box-like structure enclosing fluorescent lighting tubes behind a semi-transparent light diffusing faceplate defining a display area. Commonly, transparencies are retained on a surface of the faceplate by pushing the upper edge of the transparencies under spring-loaded film-holder clips located along the top edge of the faceplate.
Standard size illuminators have a faceplate 17 inches high and 14 inches or multiples of 14 inches (i.e. 28 inches or 56 inches) wide. Usually, each 14 inch width of faceplate has its own fluorescent tubes and control switch. Such faceplates enable viewing full size X-ray films which measure 17 inches by 14 inches. In such cases, the sections of the faceplate not covered by transparencies need not be illuminated. This eliminates unnecessary glare from areas outside the transparency.
When transparencies smaller than 14 inches by 17 inches are to be examined, they are typically retained on the faceplate area in the same manner as full size transparencies, i.e., suspending them by means of the film-holders along the top of the viewer. This leaves a portion of the faceplate area surrounding the transparencies fully illuminated and the resulting additional glare detracts from the visual perception of the person trying to study the image and assess the information it contains.
Often, transparencies contain several very transparent areas, and frequently, radiologists have to examine over-exposed transparencies. In these cases, considerable glare emanates through transparent portions of the transparencies themselves, and from the areas surrounding the transparency. Glare causing portions can especially be found in collimated images, multi-exposure images and in certain anatomical regions, such as for example in a chest image, wherein the abdomen portion of the image is very transparent and may interfere with detecting small lesions in the lower part of the lungs.
An important factor in the interpretation of transparencies, is contrast resolution (the ability to discriminate between various levels of light). This ability is determined by Weber's Law. E. H. Weber found that "the minimum perceptible difference in a stimulus is proportional to the level of the stimulus". Stated in terms of vision, as formulated by Fechner, .delta.L/L=K (Weber constant); Where .delta.L is the minimal detectable difference in luminance; and L is the luminance, see "Visual Psychophysics", D. Jameson and L. M. Hurvich (ed.), Berlin, 1972.
Accordingly, if the eye is adapted to luminance L, .delta.L is determined. For a radiologist, maximal gray level discrimination is desired. Therefore, the observer's eye should be adapted to the luminance level of the image under study. In less benign conditions, a person reading an X-ray image will be less able to perceive critical but minor shadings and nuances in the transparency. Moreover, protracted inspection of faceplate areas under less benign conditions involves significant eye strain on the part of the observer.
While it is of course feasible for an observer to overlay masking strips on the faceplate area and on the transparency and thus block unwanted and contrast-reducing light passing through the faceplate area, as a practical matter, readers of X-rays rarely resort to such practice.
Another important factor in the interpretation of transparencies is the intensity of the backillumination. As is known in radiology psychophysics, lesion delectability is optimized when the Luminance level emerging from the transparency is between 200 and 500 cd/meter.sup.2, i.e., about 100 nit.
Mammograms are among the most challenging x-ray film transparencies to interpret. One difficulty with viewing mammograms is that the densities of the images are relatively high, so that an intense backillumination is required to achieve an optimum acuity of the eye of the observer of the transparency. Further, masking mammograms is difficult and time consuming. For example, the side of the image nearer the chest contains clinically important details even at the edge of the transparency. Masking which overlaps the transparency results in loosing image content, while spacing the masking from the transparency results in glare which might dazzle the reader. Another difficulty with viewing mammograms is that the size of some of the lesions of interest are very small (micro calcifications are 50-150 microns) on one hand and some of the lesions have a low density on the other hand, so that the backillumination must be artifact free.
An X-ray viewer is typically suitable for reading a wide variety of transparencies. However, this may be a limitation rather than a benefit for a radiologist who specializes in one particular field. A specializing radiologist may be required to use a viewbox which is not optimal for his particular need and, furthermore, the viewbox may have many complicated features which are not necessary for his work. A mammogram specialist, views films which come only in two sizes: 18.times.24 cm and 24.times.30 cm. A nuclear medicine or ultrasound specialist on the other hand views images which are 8.times.10 inches.
This application is a further improvement on a previously filed series of applications for improved display devices for transparencies. These applications, the disclosures, claims, annexes, appendices and drawings of which, if any, are incorporated herein by reference are U.S. Pat. No. 5,430,964 which was filed on Jun. 30, 1992 as U.S. application Ser. No. 07/861,982, a U.S. Provisional application number 60/007,522, entitled "Improved Display Device" filed Nov. 24, 1995 by Inbar, et al., PCT application PCT/EP95/04693 filed Nov. 27, 1995, entitled "Improved Display Device", published as WO96/17269 and PCT application PCT/EP94/03968, published as WO95/14949. The PCT applications name, inter alia, the United States of America as a designated state. The above referenced applications describe methods of masking, back-illuminating and image processing algorithms generally preferred for the practice of the present invention. Israel patent application 114,911, titled "Backprojection Transparency Viewer", filed Aug. 11, 1995, and corresponding PCT application PCT/IL96/00026, the disclosures of which are incorporated herein by reference, describe apparatus useful for a backprojection viewbox and which are also preferred for some embodiments of the present invention. U.S. provisional application No. 60/001814 "Transparency Viewing Apparatus", filed Aug. 1, 1995, its corresponding Israeli application 114,258 of like title filed Jun. 21, 1995, and PCT application PCT/IL96/00023, the disclosures of which are incorporated herein by reference, describe light recycling methods especially suitable for the practice of preferred embodiments of the present invention.